The invention relates to an optical device for delivering a single radiation beam, having a phase-locked diode laser array and a collimator. The lens laser array radiates in a stable supermode having, in a lateral plane through the axis of symmetry of the radiation and parallel to the pn-junction layer of the laser array, two far field lobes. The invention also relates to an apparatus for scanning an information medium, such as an optical disc recorder and/or player or a laser printer, comprising such optical device.
The paper "Phased Array Diode Lasers" in "Laser-Focus/Electro Optics", June 1984, W Streifer et al, describes a phased-locked diode laser array which is a special kind of multi-emitter semiconductor laser. There is an increasing interest in phase-locked diode lasers because they can deliver a considerably higher output power than a single-emitter diode laser. For many applications it is necessary to focus the output power of all of the emitters into a single, circularly and diffraction-limited spot with as high efficiency as possible. Such focussing cannot be carried out by a high-aperture objective lens solely, because of the specific radiation pattern of the phase-locked laser array.
Such an array with N emitters radiates in N so-called supermodes, and the laser itself selects that or those, supermode(s) for which its threshold current is minimum. The threshold current is the value of the electric current perpendicular to the p-n-junction layer at which the laser action starts. The specific internal structure of a diode laser array determines the favoured radiation mode of this array.
As disclosed in the mentioned paper in "Laser-Focus/Electro Optics", June 1984, the so-called highest order supermode is, may be the favoured one. In this mode the radiation of each emitter is .pi. radians out of phase with that of its two neighbour emitters. Because of this phase shift the highest order supermode is stable up to high power. The diode laser array may also radiate in other supermodes which are also stable. In general a stable supermode is one for which the radiation of some emitters have a first phase which is always .pi. radians different from a second phase which the radiation of the other emitters have at that time.
In the theory of diode laser arrays the terms "near field phase- or intensity distribution" and "far field phase- or intensity distribution" are used. The former is the distribution at the position of the emitting facet of the diode laser array, or in a plane conjugate to this facet. The far field phase- or intensity distribution is the result of the interference of the individual beams emanating from the individual slit-shaped emitters of the array. The far field distribution is formed at some distance from the emitting facet. The near-and far field in this application are reserved for those fields in the lateral plane, i.e. the plane through the axis of the radiation pattern and parallel to the pn-junction layer. The plane perpendicular to the pn-junction layer and through the axis is the transversal plane.
The highest order and other stable supermode have two radiation lobes in the far field. The paper of W. Streifer et al does not disclose how a single beam, suited for forming a single radiation spot, can be obtained from a diodelaser array radiating in these modes.